Though I do like the point about not judging by human standards; I mean, dolphins do it because they enjoy it, and because no dolphin in history has ever looked at another and said 'You faggot!' Now what makes humans so much less clever?

Andy17 nails it

Nice publicity stunt, guys, though I do have to admit it might be a little more practical in London than in the US, where we actually have enough space for things like highways.

(Oh, and to the one anonymous coward who talks about burning coal at a power station vs. gasoline in a car -- no, no, no, you've got it all wrong. The term is "carbon offset". Can you say "carbon offset"? I knew you could!)